A combination of climate change and pollution is chewing through Europe's ozone, researchers say. They have announced that the protective layer over northern and central Europe was thinner this season than it has been since measurements began 50 years ago.

The results come from a campaign that collected ozone data from 35 stations from Greenland to Tenerife, between January and March 2005. Preliminary analysis of these data plus information from satellites reveals that about one-third of ozone molecules in the arctic stratosphere were destroyed this past winter. By early spring, ozone-depleted air had drifted southwards through large parts of northern and central Europe.

Substantial ozone depletion was observed during several cold arctic winters in the 1990s, most notably in 1999/2000. But this year's reduction of 30% is larger than seen before, scientists said at this week's meeting of the European Geosciences Union in Vienna.

Some, but not all, of this loss was replenished by ozone flooding up from the south. In fact, the loss came closer to creating a fully fledged ozone hole than ever before, says Markus Rex, an atmospheric scientist at the Alfred Wegener Institute of Polar and Marine Research in Potsdam, Germany.

Without a healthy layer of ozone for protection, light-skinned Europeans can get sunburned, even in the spring, in 20 minutes, particularly at high altitudes or when snow reflects the sunlight. Researchers are concerned that the radiation may have more dramatic effects on plants and animals, which cannot shield themselves with sunscreen.

Out in the cold

Ozone is destroyed when oxygen molecules react with aggressive chemicals produced by the decay of chlorofluorocarbons (CFCs). The use of CFCs was largely phased out by the 1987 Montreal Protocol. But CFCs are long-living substances that will continue to destroy atmospheric ozone in polar regions for at least another 50 years.

Climate change appears to worsen their effect. High-altitude clouds made of nitric acids, sulphuric acid and water trigger the rapid transformation of CFCs into more aggressive compounds. And unusually cold arctic winters, which are expected to become more frequent as global surface temperatures rise, seem to favour the formation of such clouds.

"Over the last 40 years or so, we have seen a fourfold increase in the area cold enough for polar stratospheric clouds to form throughout the Arctic" says Rex. The reasons are not yet entirely understood.

Researchers knew it was going to be a particularly cold year in the Arctic this January, and their predictions of severe ozone depletion have now been confirmed.

To work out the fate of the arctic ozone layer in the more distant future, scientists will need a better understanding of how greenhouse gases affect temperatures in the upper atmosphere, says Neil Harris an atmospheric chemist at the University of Cambridge, UK. Current models of the stratosphere predict everything from a dramatic cooling in the Arctic to modest warming.